[BUGFIX] Fix vocab determinism in py35 (#1166)

[eric-haibin-lin]

Description

Backport #1166

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

cc @dmlc/gluon-nlp-team

[codecov]

Codecov Report

Merging #1167 into v0.9.x will increase coverage by 6.57%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           v0.9.x    #1167      +/-   ##
==========================================
+ Coverage   81.93%   88.51%   +6.57%     
==========================================
  Files          72       72              
  Lines        6976     6976              
==========================================
+ Hits         5716     6175     +459     
+ Misses       1260      801     -459

Impacted Files	Coverage Δ
src/gluonnlp/model/transformer.py	91.66% <0%> (+0.32%)	⬆️
src/gluonnlp/model/attention_cell.py	91.06% <0%> (+0.55%)	⬆️
src/gluonnlp/data/batchify/batchify.py	95.27% <0%> (+0.78%)	⬆️
src/gluonnlp/data/transforms.py	83.05% <0%> (+1.66%)	⬆️
src/gluonnlp/model/bert.py	94.62% <0%> (+2.98%)	⬆️
src/gluonnlp/data/datasetloader.py	85.34% <0%> (+3.01%)	⬆️
src/gluonnlp/data/corpora/wikitext.py	100% <0%> (+5.17%)	⬆️
src/gluonnlp/model/parameter.py	100% <0%> (+8%)	⬆️
src/gluonnlp/data/dataset.py	98.63% <0%> (+10.2%)	⬆️
src/gluonnlp/data/word_embedding_evaluation.py	89.31% <0%> (+16.41%)	⬆️
... and 7 more

[mli]

Job PR-1167/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1167/2/index.html